Changes in the Sea of Okhotsk
due to global warming− Weakening pump function to the North Pacific −Kay I. Ohshima
Institute of Low Temperature Science

In recent years, the amount of sea ice in the Arctic
Ocean has significantly decreased during the summer
months, and several models predict that it will
eventually disappear in summer altogether. Recentresearch has revealed that global warming is also
affecting the Sea of Okhotsk, which is also an icecovered sea.

Southern limit of sea ice in the NorthernHemisphereSea ice comes ashore every winter along the Seaof Okhotsk coast of Hokkaido, which is the only
place where sizable sea ice can be observed at thislatitude. The white areas of Fig. 1 indicate the average
distribution of sea ice in February. The figure showsthat the Sea of Okhotsk is the southern limit of sea ice
in the Northern Hemisphere.
Why does the Sea of Okhotsk represent this limit?
Figure 1 shows average February air temperaturesusing colored isograms. We can see that the cold pole(i.e. the coldest place) in the Northern Hemisphere islocated in the upwind region of the Sea of Okhotsk.
The biggest factor contributing to this sea’s status as
the southern limit is the extremely cold air from thecold pole that blows over it from autumn to winter.

Sea of Okhotsk produces the densest
water in the North Pacific
When sea ice is formed, most of the salt content isrejected from the ice and thus cold and saline water isreleased into the ocean below. The colder the seawateror the higher its salt content, the denser it becomes.

Since large amounts of sea ice are formed in the Seaof Okhotsk, the densest water in the North Pacific (orto be exact, the densest water on the surface of theNorth Pacific) is produced there. Large-scale verticalcirculation (convection) involving the ocean’s deep andintermediate layers is driven by density differences:
water sinks in the dense water formation area andthen gradually upwells in other areas. Although
the dense water that can reach the deep layer is notproduced in the North Pacific, the dense water thatreaches the intermediate layer (approx. 200 to 800m deep) can be produced by the sea ice formationin the Sea of Okhotsk. Sinking of this dense water
creates the vertical circulation (i.e. convection) down
to the intermediate depths in the North Pacific scale.
The Sea of Okhotsk thus plays a role as the heartpump of the North Pacific. As such, the cold, densewater produced in the Sea of Okhotsk sinks into
the intermediate layer, spreading through the Kuril
Straits, then into the whole area of the North Pacific’s
intermediate layer.

Global warming and the Sea of Okhotsk
Over the last three decades since accurate observation
by satellite became possible, sea ice extent in the Seaof Okhotsk has decreased by approximately 150,000
km2, corresponding to about 10% of the entire area of
the Sea of Okhotsk (blue line in Fig. 2). It has been
also revealed that yearly variability of sea ice extent
in the Sea of Okhotsk is highly correlated with thatof surface air temperatures in the upwind region of
the Okhotsk (red line in Fig. 2). Of particular noteis that this temperature has risen by approximately

Fig. 1 Climatology of global sea ice distribution(white) and surface air temperatures (isograms)in February.

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Fig. 3  Time series of temperature(red line) and dissolved oxygen(blue line) of the intermediatewater in the Sea of Okhotsk during
the past 50 years.   Comparison
was made at a certain density
level, corresponding to approx.
500 m deep.

Fig. 2  Time series of sea ice extent anomaly in the Sea ofOkhotsk in February (blue line) and surface air temperatures
anomaly in the upwind region of the Okhotsk (red line).
Note that the scale of the sea ice extent is inverted (the axison the right).  Surface air temperatures are the mean values
between October and March.

2.0°C over the past 50 years. This value of 2.0°C
far exceeds the rate of average temperature increase
worldwide (0.74°C over the past 100 years), thereby
clearly indicating that the region is significantly
affected by global warming. The correlation between
this temperature and the sea ice extent suggests that
decreases in the sea ice preceded the beginning ofsatellite observations.

Sea ice reduction weakens overturning inthe North Pacific
If sea ice production decreases, does the level of densewater production also decline? Analysis performed by
combining the latest ocean data obtained by Hokkaido
University and other institutions with past data pointed
to significant changes: the water temperature in theintermediate layer of the Sea of Okhotsk has increased
over the past five decades, while the dissolved oxygen
concentration has decreased (Fig. 3). This means thatthe sinking of cold oxygen-rich surface water into theintermediate layer has decreased. That is to say, thedecrease of sea ice production causes the decrease incold, dense water production.
Recent research has also revealed that since thesinking of cold, dense water generated in the Sea ofOkhotsk causes the overturning down to the middepth of the North Pacific, the influence of changes
in the Sea of Okhotsk reaches as far as circulation inthe North Pacific. Figure 4 indicates trends in watertemperature in the intermediate layer of the North
Pacific, including the Sea of Okhotsk, over the past
50 years. The warming trend is the most prominent
in the Sea of Okhotsk, and the area of warming signal
has spread to the North Pacific along the pathway ofthe seawater flowing out of the Sea of Okhotsk (greenline in Fig. 4). This suggests that the reduction in
the amount of water sinking in the Sea of Okhotsk
weakens the overturning in the North Pacific.

Further scenarios
In a nutshell, the Sea of Okhotsk is highly sensitive

to global warming: over the past 50 years, the levelof sea ice production has decreased and the amountof dense water sinking has thus declined, thereby
weakening the overturning in the North Pacific. To
put it simply, recent global warming has weakened theSea of Okhotsk’s workings as a pump.
The weakened overturning has various implications.
Of particular note is iron circulation, which isconsidered an important factor in determining
biological productivity according to recent research.
It has been recently revealed that when dense water
sinks to the intermediate layer in the Sea of Okhotsk,
iron is also brought to this layer. One hypothesis saysthat this iron is also supplied to the western area of theNorth Pacific and supports high biological productivity
there. If that is the case, it is also possible to suggestthat if global warming weakens sea ice productionin the Sea of Okhotsk, iron supplies will decrease in
the North Pacific as well as in the Sea of Okhotsk,
thus reducing levels of biological productivity andfishery resources. Hokkaido University has begun
interdisciplinary research programs transcending thefields of physics, chemistry, biology and fisheries withthe aim of verifying these hypotheses and scenarios.

Fig. 4   Trends in temperature of the intermediate water in the North
Pacific and the Sea of Okhotsk over the past 50 years.  The trends werecalculated at a certain density level, corresponding to approx. 300 - 500
m deep.

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